This invention relates to an output correction method for exhaust gas ingredient-concentration sensors adapted for use in internal combustion engines, and more particularly to a method of this kind, which utilizes a sensor formed of two oxygen concentration sensor elements having proportional output characteristics different from each other and corrects the output value of one sensor element based upon the output value of the other sensor element.
Electronic fuel supply control for internal combustion engines generally includes air-fuel ratio feedback control wherein the concentration of an ingredient contained in exhaust gases emitted from the engine is detected, and the air-fuel ratio of a mixture actually supplied to the engine (hereinafter called "the actual air-fuel ratio") is controlled to a desired air-fuel ratio in a feedback manner responsive to the detected concentration value, in order to improve emission characteristics, fuel consumption, and so on. Conventional air-fuel ratio feedback control methods include methods using a proportional output type sensor as the exhaust gas ingredient-concentration sensor, which has such an output characteristic as to produce an output proportional to the concentration of an ingredient in the exhaust gases.
However, such proportional output type sensor is required to have an output accurately corresponding to the ingredient concentration for properly controlling the air-fuel ratio. However, after a long-term use, the sensor is apt to deteriorate mainly due to clogging of a gas-introducing slit thereof with oxides etc. contained in the exhaust gases, failing to maintain a desired output characteristic. However, the deterioration of the sensor cannot be easily detected by the use of a single sensor element.
To solve the above problem, a method of correcting the output value of an exhaust gas ingredient-concentration sensor has been proposed, e.g., by Japanese Provisional Patent Publication (Kokai) No. 62-201346, which uses a sensor formed of two oxygen concentration sensor elements having different output characteristics from each other, and corrects the output value of one sensor element based upon an output value of the other sensor element when a predetermined operating condition of the engine is satisfied, i.e., when the engine is in a stable operating state.
However, the proposed method has the disadvantage that correction of the sensor output characteristic cannot be accurately effected when the engine is operating in particular operating regions. Specifically, in general, when the engine is in a stable operating state, the actual air-fuel ratio can be controlled to a desired air-fuel ratio or its approximal ratio. However, there can be cases where the engine operation is stable even when the actual air-fuel ratio is considerably different from the desired air-fuel ratio. For example, when there is a large amount of fuel adhering to the inner wall of the intake pipe, the adhering fuel is entrained into the engine cylinders together with fuel normally injected into the intake pipe to thereby cause the actual air-fuel ratio to be excessively rich. Also, when the adhering fuel amount is very small, part of the mixture normally injected into the injection pipe adheres to the inner wall of the intake pipe to thereby cause the actual air-fuel ratio to be excessively lean. This tendency is remarkable particularly when the engine is restarted at a high temperature shortly after it is stopped, or when the engine is brought into a constant-speed operation after acceleration or deceleration.
However, according to the proposed method, the output correction is carried out by comparing the output value of the one sensor element with the output value of the other sensor element when the actual air-fuel ratio stays at almost a constant value, that is, when the engine is in a stable operating state. Consequently, even when there is a large difference between the actual and desired air-fuel ratios during stable engine operation, as mentioned above, it is erroneously judged that the engine is in the stable operation and accordingly the output correction is effected. On such an occasion, however, the actual air-fuel ratio varies with variation in the amount of fuel actually supplied to the engine due to increase and decrease in the amount of adhering fuel to the intake pipe, etc. thereby making it impossible to carry out exact output correction of the sensor. As a result, the air-fuel ratio is erroneously controlled by improperly corrected sensor output, which can lead to clogging of the ignition plugs, excessive increase of CO in the exhaust gases, etc. when the air-fuel ratio is controlled toward the rich side, or can lead to degradation in the driveability of the engine, etc. when the air-fuel ratio is controlled toward the lean side.
Further, according to the proposed method, the feedback control of the air-fuel ratio is interrupted while the output correction is being carried out, in order to maintain the air-fuel ratio at a constant value. Consequently, the actual air-fuel ratio is not controlled toward the desired air-fuel ratio during the sensor output correction, so that the above-mentioned state in which the difference between the actual air-fuel ratio and the desired air-fuel ratio is large continues over a long period of time, thereby leading to degradation in the emission characteristics, increased fuel consumption, etc.